I have tried this. Deleted the _Downloads folder. Re-installed the update following a repair and the same error occurs.
I don't really want to re-install the whole thing, as that takes forever and might give me the same result!
If I click later when it prompts to update it gives the error:
Can't run G:\Eagle Dynamics\DCS World Open Beta\bin/DCS.exe: (2) The system cannot find the file specified.